(1) Filed of the Invention
The present invention relates to a developer for electrophotography, and more particularly to improvement of a developer for electrophotography in its life and image stability.
(2) Description of the Related Art
Generally, as a developer for electrophotography there has been known two-component developer which is composed of a toner component that visualizes a static latent image on the photoconductor therewith, and a carrier component that generates charges by triboelectrification to impart the charges to the toner and serves to carry over the toner to development region.
With regard to such a two-component developer, the toner component typically is compounded of about 80 to 90% of binder resin, about 5 to 15% of a colorant, about 1 to 5% of electrification inhibitor, and some percent or less of surface lubricant and/or other additives.
On the other hand, with regard to the carrier, there are known iron powder type carriers, ferritic carriers and binder type carriers, etc. Of these, ferritic carriers are widely used. These ferritic carriers are generally coated with resin by spraying, immersion or other techniques. Examples of such resins for coating include polyester resins, fluorine-contained resins, acrylic resins and silicone resins, etc., and one or more kinds of resins are used for this purpose.
Meanwhile, if a developer, and therefore, its carriers also, has undergone repeated copying operations, the coatings of such carriers generally tend to be peeled off, and to make matters worse, the toner particles are liable to adhere or stick to the carriers due to forces except those caused by charges, for example, by mechanical forces. These phenomena cause, for example, the image density to extremely lower, thus presenting a great difference or degradation to the total image features, compared to images copied when the developer was fresh.
In view of shape and feature of carrier particle itself, when cores of carrier particles has smaller specific surfaces, the surface of the core particle is relatively smooth. Therefore, coating made on such a core becomes so soft and delicate without much toughness, that the coating is easy to be peeled away in the agitation process. Accordingly, resistance of the total developer lowers, and even if the developer is highly charged, the charges will leak away. For this reason, the charging condition and the image density are stabilized while the life is disadvantageously short.
In contrast, a carrier for electrostatic charge development with a core member having a large specific surface has been proposed in, for example, Japanese Patent Application Laid-Open hei-4 No. 83264. The carrier disclosed is designed to improve resistance of the carrier to pollution, and comprises a core member of irregular ferrite with a specific surface of 300 cm.sup.2 /g or more, and fluoro-resin mainly containing monomer vinylidene fluoride coated thereon. Carriers with a core having such a great specific surface form a firm coating thereon, since the core particle is complicatedly jagged. Accordingly, repeated agitations will not make the coating peel away, as compared with the case where cores have a smaller specific surface. Therefore, the life of the carrier is prolonged while the developer is highly charged to disadvantageously make the image unstable and its image density low.
Thus, in the case where the same developer is used repeatedly over a prolonged period of time, it is very difficult to achieve stable image density and, at the same time, make longer the life of the developer when a conventional developer is used for copying process. In other words, the object can be only achieved by a developer that contains carriers presenting stable charging properties and no deterioration of such as coat-peeling or the like.